1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device implementing in-plane switching (IPS) where an electric field to be applied to liquid crystal is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A typical liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientational order in alignment resulting from their thin and long shapes. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field to the liquid crystal molecules. In other words, as the alignment direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Since the incident light is refracted to the orientation of the liquid crystal molecules due to the optical anisotropy of the aligned liquid crystal molecules, images are displayed.
Generally, typical LCD devices include upper and lower substrates with liquid crystal molecules interposed therebetween. The upper and lower substrates are generally referred to as color filter and array substrates, respectively. The upper and lower substrates respectively include electrodes disposed on opposing surfaces of the upper and lower substrates. An electric field is generated by applying a voltage to the electrodes, thereby driving the liquid crystal molecules to display images depending on light transmittance.
Of the different types of known LCDs, active matrix LCDs (AM-LCDs), which have thin film transistors and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images. Driving methods for such LCDs typically include a twisted nematic (TN) mode and a super twisted nematic (STN) mode.
However, the operation mode of the TN- or STN-LCD panel has a disadvantage of a narrow viewing angle. That is to say, the TN liquid crystal molecules rotate with polar angles 0 to 90 degrees, which are too wide. Because of the large rotating angle, contrast ratio and brightness of the TN- or STN-LCD panel fluctuate rapidly with respect to the viewing angles.
To overcome the problem, an in-plane switching (IPS) LCD panel was developed. The IPS-LCD devices typically include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates. Therefore, the IPS-LCD panel implements a parallel electric field that is parallel to the substrates, which is different from the TN- or STN-LCD panel and has advantages in contrast ratio, gray inversion, and color shift that are related to the viewing angle.
A detailed explanation about operation modes of a typical IPS-LCD device will be provided with reference to FIGS. 1 to 5.
As shown in FIG. 1, upper and lower substrates 1 and 2 are spaced apart from each other, and a liquid crystal 3 is interposed therebetween. The lower and upper substrates are called array and color filter substrates, respectively. Pixel and common electrodes 4 and 5 are disposed on the lower substrate 2. The pixel and common electrodes 4 and 5 are parallel with and spaced apart from each other. A color filter 7 is disposed on a surface of the upper substrate 1 and opposes the lower substrate 2. The pixel and common electrodes 4 and 5 apply an electric field 6 to the liquid crystal. The liquid crystal has a negative dielectric anisotropy, and thus it is aligned parallel with the electric field 6.
FIGS. 2 to 5 conceptually illustrate operation modes of a typical IPS-LCD device. When there is no electric field between the pixel and the common electrodes 4 and 5, the long axes of the liquid crystal molecules 3 maintain an angle, for example, the angle is 45 degrees, from a line perpendicular to the parallel pixel and common electrodes 4 and 5 as shown in FIG. 3. On the contrary, when there is an electric field between the pixel and common electrodes 4 and 5, there is an in-plane electric field 6 parallel to the surface of the lower substrate 2 between the pixel and common electrodes 4 and 5 because the pixel and common electrodes are formed on the lower substrate 2 as shown in FIG. 4. Accordingly, the liquid crystal molecules 3 are twisted so as to align the long axes thereof in the direction of the electric field, thereby being aligned such that the long axes thereof are parallel with the line perpendicular to the elongated direction of the pixel and common electrodes 4 and 5 as shown in FIG. 5. By the above-mentioned operation modes and with additional parts such as polarizers and alignment layers, the IPS-LCD device displays images. The IPS-LCD device has a wide viewing angle and low color dispersion characteristic. Specifically, the viewing angle of the IPS-LCD device is about 70 degrees in direction of up, down, right, and left. In addition, the fabricating processes of this IPS-LCD device are simpler than other various LCD devices.
FIG. 6 is a schematic plan view of an array substrate of the typical IPS-LCD device.
As shown, a pixel area is defined by a row gate line 11 and a column data line 41. A TFT “T”, the switching device, is formed at the crossing of gate and data lines. In the pixel area, a common line 15 is elongated along the direction of the gate line 11 and a plurality of common electrodes 16 connected to the common line 15 are elongated along the direction of the data line 41. Moreover, in the pixel area, a plurality of pixel electrodes 43, which are spaced apart from the common electrodes 16 and arranged in an alternating pattern, is connected to the TFT “T” and the pixel line 45. The pixel line 45 overlaps the gate line 11 to make a storage capacitor “S”.
Therefore, in the IPS-LCD devices, since lateral electric field is formed between the common electrodes 16 and the pixel electrodes 43 of the same plane and the liquid crystal molecules are aligned parallel to the lateral electric field, the viewing angle can be improved. Furthermore, the IPS-LCD devices have low color dispersion qualities and the fabricating processes thereof are simpler than those of other various LCD devices.
However, because the common and pixel electrodes 16 and 43 are disposed on the same plane on the lower substrate, the transmittance and aperture ratio are low. In addition, a response time according to a driving voltage should be improved and a cell gap should be uniform because of the low alignment margin. A color shift problem according to the viewing angle still remains. These problems are dependent on the rotational direction of the liquid crystal molecules under the electric field over the threshold voltage and are generated from the increase or decrease of the retardation and of the liquid crystal layer according to the viewing angle.
FIG. 7 is a schematic plan view of an array substrate of the IPS-LCD device for solving the color shift problem.
As shown, upper and lower domains “A” and “B” are formed by bending the common and pixel electrodes 16 and 43 at an angle with respect to the common line 15. The electric field between two electrodes 16 and 43 rotates the liquid crystal molecules 81 and 82 of the domains “A” and “B” in opposite direction from each other. A liquid crystal molecule of the upper domain “A” is rotated clockwise and a liquid crystal molecule of the lower domain “B” is rotated counter-clockwise. Therefore, the liquid crystal molecules 81 and 82 of two domains “A” and “B” are aligned in different directions to compensate the color shift effectively.
Here, since the data line 41 is also bent at an angle with respect to the common line 15 and is patterned parallel to the common and pixel electrodes 16 and 43, the space between the data line 41 and the common electrode 16 can decrease, and the aperture ratio can be improved. To make the most of these advantages, a black matrix of an upper substrate also should have a bent portion. However, in the IPS-LCD device, since the metallic black matrix affects the voltage between the common and pixel electrodes 16 and 43, the black matrix is made of resin, which cannot be formed with a bent portion because of the limit of the processing technology. Therefore, the IPS-LCD device of FIG. 7 has a limit for effective realization.
FIGS. 8A to 8D are sequential cross-sectional views taken along a line “VIII—VIII” of FIG. 7 showing the fabrication process for the array substrate of the typical IPS-LCD device.
FIG. 8A shows the step of patterning gate electrode 12, common and storage electrodes 16 and 11 of a first metal layer, which can be made of metal, for example, aluminum (Al) or chromium (Cr), on the substrate 10.
FIG. 8B shows the step of forming a gate insulator 21 and patterning an active layer 23 and an ohmic contact layer 25 on the first metal layer. The gate insulator 21 can be made of silicon nitride (SiNx) and the ohmic contact layer 25 is doped by impurities.
FIG. 8C shows the step of patterning another storage electrode 45 and source 47, drain 49, pixel 43, electrodes and data line 41, of a second metal layer. The source and drain electrodes 47 and 49 are patterned on the ohmic contact layer 25 and the pixel electrodes 43 are spaced apart from the common electrodes 16 on the gate insulator 21.
FIG. 8D shows the step of forming a passivation layer 51, which prevents the active layer 23 from contamination of mists or impurities, on the entire surface of the substrate.